Storm Forecast
Valid: Mon 17 Jun 2013 06:00 to Tue 18 Jun 2013 06:00 UTC
Issued: Sun 16 Jun 2013 21:31
Forecaster: GROENEMEIJER

A level 1 was issued for SW France and NE SPain mainly for large hail.

A level 1 was issued for W France, central France, NW France, the BeNeLux countries mainly for large hail, and to a lesser extent for tornadoes and severe wind gusts.

A level 1 was issued for eastern France parts of Germany, the Alpine countries, N Italy and the W Balkans mainly for large hail and to a lesser extent for severe winds.

A level 1 was issued for central Italy mainly for large hail and to a lesser extent for severe winds.

SYNOPSIS

Monday morning a mid-level low will be cutting off from the westerlies reaching the NW of the Iberian Peninsula on Tuesday morning. The system is sourrounded by a strong cyclonically curved mid-level jet with wind speeds in excess of 35 m/s at 500 hPa on its southern and eastern flanks. Along an axis from the southern UK toward the Alps, a ridge builds at mid levels with significant height rises. A plume of warm air is advected northward across much of western and central parts of Europe. Despite relatively strong lapse rates over western Europe and abundant humidity in some areas, a strong cap will likely prevent convective initiation in most places. Storms may however develop along the edges of the warm air and in the vicinity of mountain ranges. The extent to which this occurs is the greatest factor of uncertainty for this forecast.

Spain and SW France...

Synoptic scale lift is expected to weaken convective inhibition over over NE Spain and SW France, where scattered storms are expected to form during the late morning and afternoon. They will do so in a very strongly sheared environment with 0-6 km bulk shear of 25 up to 40 m/s. Modest CAPE values of 500-1000 J/kg should limit the overall severe threat. Scattered storms including supercells are forecast, with a primary threat of large hail and possibly severe winds. During the late afternoon and an influx of colder boundary-layer air should mitigate the storm risk across SW France by preventing surface-based convection.

NW, W and central France, BeNeLux...

Numerical models suggest that clustered convective activity will be ongoing over western France early on Monday. These storms appear to be associated with a maximum of warm air advection that is forecast to translate northeastward into the BeNeLux countries during the evening. The system, as simulated in the models is elevated and occurs on the cold side of an northward advancing surface warm front. This should mitigate the overall severe threat. Nevertheless, it is thinkable that some surface-based storms will develop as well, and benefit from strong deep layer shear (0-6 km bulk shear 25-30 m/s) and a strongly helical flow (300 m2/s2 storm-rel helicity). The environment would then be supportive of supercells with a risk of very large hail, severe winds and perhaps a tornado. Presently, this scenario is not supported by NWP models. With the elevated storms, severe hail should be the biggest threat. If confidence in the development of surface-based convection and/or in a high convective coverage increases, an update to a higher risk level may be warranted.

Eastern France, Germany, Alpine Countries, Italy, Dinaric Alps...

Some signals of convective development are present in both the GFS and ECMWF numerical models within the warm air-mass. Given that CAPE will likely rise to above 2000 J/kg in places, a isolated pulse storms are possible. These may produce large hail, and perhaps some localized downbursts with severe winds and very heavy rain. Areas where such storms may occur include the Massif Central, Jura, Alps and many of the minor mountain ranges in Germany, as well as the mountains of the Western Balkans and the Appennines. The overall storm coverage is however low, and development of larger convective systems appears unlikely.

Finland and adjacent NW Russia...

Within a cool air-mass, scattered storms are expected to form in response to diurnal heating. Given weak to moderate shear and low CAPE, no significant severe threat is expected, although some small hail and perhaps an isolated weak landspout-type tornado may occur.